1. Field of the Invention
The present invention relates to a hydraulic control apparatus for an automatic transmission that is mounted, for example, in a vehicle, and in particular, relates to a hydraulic control apparatus for an automatic transmission in which fail-safe control is carried out by switching a switching valve to a fail position based on a signal pressure of a fail solenoid valve.
2. Description of the Related Art
Conventionally, a staged automatic transmission that is mounted, for example, in a vehicle, enables multi-speed shifting by controlling the engagement state of a plurality of friction engagement elements (clutches and brakes) by using a hydraulic control apparatus and establishing transmission paths in the speed change mechanism at each speed. Such a hydraulic control apparatus is provided with a plurality of solenoid valves that regulate and output engagement pressures to the respective hydraulic servos that engage the plurality of friction engagement elements, and the control of the multiple-speed shifting is carried out by engaging the friction engagement elements that are necessary for establishing each of the shift speeds by the electronic control of these solenoid valves (refer to Japanese Patent Application Publication No. JP-A-8-42681 and Japanese Patent Application Publication No. JP-A-2000-240776).
However, in the hydraulic control apparatus described above, providing a switching valve that is set to a fail position by switching a spool position only during a failure so as to carry out fail-safe control when some sort of failure (damage) has occurred can be considered. An example of using such a switching valve is providing a structure in which the switching valve is switched to enable supplying an engagement pressure to a prescribed hydraulic servo by bypassing the solenoid valve when some sort of failure has been detected in the hydraulic control apparatus and an all-solenoids-off state occurs, that is, a state when no electrical signal is sent to the solenoid valves. Thereby, the travel of the vehicle can be ensured by establishing a prescribed shift speed.
However, in order to switch the switching valve described above during a failure, it is necessary to provide a solenoid valve that is dedicated to the occurrence of failures and outputs a signal pressure during a failure that is different from the one output during normal operation, and this is a problem since these requirements become an obstacle when attempting to reduce the size and the cost of the hydraulic control apparatus.